[analyzer] Refactor RegionStore's sub-region bindings traversal.

This is going to be used in the next commit.
While I'm here, tighten up assumptions about symbolic offset
BindingKeys, and make offset calculation explicitly handle all
MemRegion kinds.

No functionality change.
Daniel Malea caught an issue where calling dotest.py with an invalid directory would cause the progressbar init code to raise an exception
This commit fixes it
[analyzer] Use collectSubRegionKeys to make removeDeadBindings faster.

Previously, whenever we had a LazyCompoundVal, we crawled through the
entire store snapshot looking for bindings within the LCV's region. Now, we
just ask for the subregion bindings of the lazy region and only visit those.

This is an optimization (so no test case), but it may allow us to clean up
more dead bindings than we were previously.
[analyzer] Tweak LazyCompoundVal reuse check to ignore qualifiers.

This is optimization only; no behavioral change.
[analyzer] Scan the correct store when finding symbols in a LazyCompoundVal.

Previously, we were scanning the current store. Now, we properly scan the
store that the LazyCompoundVal came from, which may have very different
live symbols.
[analyzer] Cache the bindings accessible through a LazyCompoundVal.

This means we don't have to recompute them all later for every
removeDeadSymbols check.
Re-apply "[analyzer] Model trivial copy/move ctors with an aggregate bind."

...after a host of optimizations related to the use of LazyCompoundVals
(our implementation of aggregate binds).

Originally applied in r173951.
Reverted in r174069 because it was causing hangs.
Re-applied in r174212.
Reverted in r174265 because it was /still/ causing hangs.

If this needs to be reverted again it will be punted to far in the future.